locking/mutexes: Delete the MUTEX_SHOW_NO_WAITER macro
MUTEX_SHOW_NO_WAITER() is a macro which checks for if there are
"no waiters" on a mutex by checking if the lock count is non-negative.
Based on feedback from the discussion in the earlier version of this
patchset, the macro is not very readable.
Furthermore, checking lock->count isn't always the correct way to
determine if there are "no waiters" on a mutex. For example, a negative
count on a mutex really only means that there "potentially" are
waiters. Likewise, there can be waiters on the mutex even if the count is
non-negative. Thus, "MUTEX_SHOW_NO_WAITER" doesn't always do what the name
of the macro suggests.
So this patch deletes the MUTEX_SHOW_NO_WAITERS() macro, directly
use atomic_read() instead of the macro, and adds comments which
elaborate on how the extra atomic_read() checks can help reduce
unnecessary xchg() operations.
Signed-off-by: Jason Low <jason.low2@hp.com>
Acked-by: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: akpm@linux-foundation.org
Cc: tim.c.chen@linux.intel.com
Cc: paulmck@linux.vnet.ibm.com
Cc: rostedt@goodmis.org
Cc: davidlohr@hp.com
Cc: scott.norton@hp.com
Cc: aswin@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1402511843-4721-3-git-send-email-jason.low2@hp.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index dd26bf6de..4bd9546 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -46,12 +46,6 @@
# include <asm/mutex.h>
#endif
-/*
- * A negative mutex count indicates that waiters are sleeping waiting for the
- * mutex.
- */
-#define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) >= 0)
-
void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
@@ -483,8 +477,11 @@
#endif
spin_lock_mutex(&lock->wait_lock, flags);
- /* once more, can we acquire the lock? */
- if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1))
+ /*
+ * Once more, try to acquire the lock. Only try-lock the mutex if
+ * lock->count >= 0 to reduce unnecessary xchg operations.
+ */
+ if (atomic_read(&lock->count) >= 0 && (atomic_xchg(&lock->count, 0) == 1))
goto skip_wait;
debug_mutex_lock_common(lock, &waiter);
@@ -504,9 +501,10 @@
* it's unlocked. Later on, if we sleep, this is the
* operation that gives us the lock. We xchg it to -1, so
* that when we release the lock, we properly wake up the
- * other waiters:
+ * other waiters. We only attempt the xchg if the count is
+ * non-negative in order to avoid unnecessary xchg operations:
*/
- if (MUTEX_SHOW_NO_WAITER(lock) &&
+ if (atomic_read(&lock->count) >= 0 &&
(atomic_xchg(&lock->count, -1) == 1))
break;